A process for separating a silicic compound from a spent steel-cleaning bath is known from application Ser. No. EP-0,141,034.
In this process the spent cleaning bath passes through a filter consisting of a material adsorbing the silicic compound.
Such a process has the disadvantage of requiring an operation of regeneration of the adsorbent materials forming the filter. In fact, in the course of its use, the adsorbent filter becomes blocked and the flow rate and the volume of the treated cleaning bath decreases progressively.
The cleaning baths currently employed for cleaning steel substrates contain iron ions, essentially in the form of ferrous ions, and a certain proportion of silicic compounds formed from the element silicon originating from the substrate.
After having been concentrated beforehand, these cleaning baths are decomposed thermally by known methods and are, in particular, calcined in an oven of the Ruthner type, and this makes it possible to recover iron oxides, essentially in the Fe.sub.2 O.sub.3 form.
The iron oxides, being sufficiently pure, can be employed as colorants, for example in paints and cosmetic products, or as excipients in the pharmaceutical industry. They also form part of the manufacture of magnetic components or of the manufacture of cements, to improve the mechanical characteristics.
The silica content of the iron oxides currently recovered from cleaning baths is at least 350 ppm, whereas it would be desirable to lower it below 100 ppm for the applications referred to above.